This invention relates to an improvement of a fuel cell.
A fuel cell so far usually used, for example, an acid electrolyte-type fuel cell comprises a pair of electrodes, i.e. an oxygen electrode 1 and a hydrogen electrode 2, and an electrolyte-containing matrix 3 provided between the electrodes, as shown in FIG. 1 (U.S. Pat. No. 4,115,627). In a practical application, a plurality of the unit cells of such structure are stacked one upon another through separator plates 4 in view of the desired output voltage.
The oxygen electrode 1 and the hydrogen electrode 2 are made from porous substrates, and gas channels, that is, an oxygen channel 1a and a hydrogen channels 2a, are provided on the respective electrode surfaces, particularly on the electrode surfaces that do not face the matrix 3, whereas catalyst layers 1b and 2b made from a porous material are provided on the respective electrode surfaces that face the matrix 3.
The fuel cell of such a structure as described above is operated by supplying oxygen molecules O.sub.2, for example, in the form of air, as an oxidizing agent to the oxygen channel 1a on the oxygen electrode and hydrogen molecules H.sub.2 as a fuel gas to the hydrogen channel 2a on the hydrogen electrode, and conducting their reaction through the catalyst layers, thereby generating electricity.
In this case, there would be no particular problem, so long as these gases such as hydrogen molecules and oxygen molecules thoroughly contact the catalysts to undergo the reaction, but the reaction has not be carried out satisfactorily on the following grounds, and consequently satisfactory performance of the fuel cell, particularly satisfactory electrode performance, has not been fully obtained.
That is, as shown in FIG. 2, where the essential parts of the fuel cell of FIG. 1 is schematically given as enlarged, a portion of hydrogen molecules H.sub.2 passing through the hydrogen channel 2a are diffused through the hydrogen electrode 2 and are ionized into hydrogen ions H.sup.+ through the catalyst layer 2b. The hydrogen ions migrate through the electrolyte-containing matrix 3 and reach the catalyst layer 1b. On the other hand, a portion of oxygen molecules O.sub.2 passing through the oxygen channel 1a diffuse through the oxygen electrode 1 and the catalyst layer 1b. At a catalyst 1c, which is wet with the electrolyte in the catalyst layer 1b, the diffused oxygen molecules and the migrated hydrogen ions undergo reaction, whereby water is generated. The thus formed water molecules H.sub.2 O in the form of water vapor diffuse through the catalyst layer 1b and the oxygen electrode 1 in the direction opposite to the diffusing direction of oxygen molecules O.sub.2, that is, in a counter-current direction thereto, and reaches the oxygen channel 1a. The water vapor joins into the remaining portion of the undiffused oxygen and is discharged to the outside of the fuel cell. In this manner, the oxygen molecules O.sub.2 and the water vapor H.sub.2 O diffuse in the counter-current direction to each other through the oxygen electrode 1 of the fuel cell of such a structure as described above, and thus the diffusion of water vapor impedes the diffusion of oxygen molecules.
The flow of gases through the oxygen electrode of the conventional acid electrolyte-type fuel cell is schematically shown in FIG. 3. That is, oxygen molecules reach the catalyst layer by diffusion and react with hydrogen ions. The thus formed water as vapor reaches the flow of oxygen, for example, in the form of air, by diffusion and is carried away thereby. Oxygen molecules and water vapor diffuse in a counter-current direction to each other, and act as resistances to their diffusion each other.
The relationship between the position of cell members and the partial pressures of water vapor and oxygen at the oxygen electrode is shown in FIG. 4, where the distribution of gas partial pressure in the oxygen electrode (air electrode) of a phosphoric acid fuel cell is shown at a current density of 500 mA/cm.sup.2. Partial pressure of oxygen is lowered to about 0.1 atm on average in the catalyst layer. A low partial pressure of oxygen at the reaction sites in the catalyst layer means a low electrode performance, and also an adverse effect on the performance of the fuel cell.
A carbon pore fuel cell, to which an oxidizing gas is supplied through a porous carbon plate without especially providing a gas channel, is disclosed in U.S. Pat. Nos. 4,125,676 and 4,129,685, but the oxidizing gas does not pass through the catalyst layer.